A student took four test tubes $\mathit{P},\mathit{Q},\mathit{R}$ and $\mathrm{S}$ and filled about $8\mathrm{ml}$. of distilled water in each. After that he dissolved an equal amount of ${\mathrm{Na}}_2{\mathrm{SO}}_4$ in $\mathrm{P},{\mathrm{K}}_2{\mathrm{SO}}_4$ in $\mathrm{Q},{\mathrm{CaSO}}_4$ in $\mathrm{R}$ and ${\mathrm{MgSO}}_4$ in $\mathrm{S}$. On adding an equal amount of soap solution and shaking each test tube well, a good amount of lather will be obtained in the test tubes: